Lithium‐Rich Solid Polymer Electrolytes Enabled by Thermal‐Assisted Dual‐Phase Dissociation for Highly Stable Flexible Electrochromic Devices

Abstract Flexible electrochromic (EC) devices with controllable optical modulation can adapt to irregular curved surfaces, offering great potential for wearable technologies and energy‐saving applications. However, for solid polymer electrolytes (SPEs) that serve as a critical component in the EC devices, challenges remain in addressing the tradeoff between their ionic conductivity and mechanical strength. Herein, a lithium‐rich solid electrolyte (LiRSE) is developed via a dual‐phase dissociation of excess LiBF 4 , involving a liquid‐phase deep eutectic solvent (DES) and a solid‐phase polyurethane polar matrix. Assisted by the thermal pressing, the DES promotes the dissociation of excess Li salt and the activation of the previously inactive crystalline regions of polyurethane to release abundant polar groups for complexation with the excess Li salt. Thus, the LiRSE shows excellent ionic conductivity of 1.14 × 10 −4 S cm −1 , which is about 3.7 times higher than the electrolyte without dual‐phase dissociation of excess LiBF 4 (3.07 × 10 −5 S cm −1 ). At last, a large‐area LiRSE film with high transmittance of 88.7% and interfacial adhesion of 0.54 MPa is fabricated to assemble flexible EC devices with a fast switching speed (2.5 s/4.5 s), long cycling stability (94.1% after 7500 cycles), and wide‐temperature stability.